You might want to focus on the way in which Macbeth responds to the increasing worsening of his situation. As the full extent of the forces ranged against him becomes clear, and as he realises how few troops have actually stayed with him, he is bestowed with a certain sense of dignity as he realises how outnumbered he is. Note the way he uses a famous image to describe his situation:
They have tied me to a stake: I cannot fly,
But, bear-like, I must fight the course.
There is nobility in the way that Macbeth determines to "fight the course" even though he is "tied... to a stake," and therefore his defeat is inevitable.
However, for me, what gives Macbeth true nobility is the way that even after he finds out that Macduff is "not borne of woman," and therefore will kill him according to the prophecies, he refuses to surrender and decides to go down fighting to try his chance against the powers of fate that have conspired against him. Consider his last speech before he is killed by Macduff:
I will not yield,
To kiss the ground before young malcolm's feet,
And to be baited with the rabble's curse.
Though Birnam wood be come to Dunsinane,
And thou oppos'd, being of no woman born,
Yet I will try the last: before my body
I throw my warlike shield: lay on, Macduff...
Macbeth recognises the fulfillment of the prophecies, but shows his nobility by refusing to fight nonetheless. He will show his customary bravery and nobility in this fight, when he knows his defeat is certain and fated.